The DTG
Reception Working Group recognised the need for a simple and
cheap test unit to be used by installers to determine whether
or not DTT reception is likely to be satisfactory at any particular
aerial outlet, and how many of the six multiplexers will be receivable.
The "blue skies" aim is for a box with six LEDs, which
would light up green, amber or red, when the signal from an aerial
is connected, and for the box to cost only a few pounds. The
unit described below doesn't meet this perhaps impossible "holy
grail", but is modestly priced and can give fairly accurate
indications of DTT receivability.

The new IMdigital meter
follows from the experience gained with the Swires IM96, widely
used by the cable TV installers.

A key requirement was for a unit,
which was simple to operate by non-technical personnel. However,
at the same time giving accurate readings. The complete tuner
was built in a screened compartment as an integral part of the
main processor board. This reduced the interconnection cost.
By using a microprocessor based design the linearity of the measuring
circuits is corrected in the software, avoiding complex alignment
procedures. New challenges have emerged with the introduction
of digital television. Merely measuring the level of a digital
signal is not sufficient. The quality of the signal must be shown
together with an adequate margin before one can be confident
of a 'no-recall' situation.

An accurate measurement of signal
levels is very important to ensure that an adequate signal level
is available on the aerial, it is also important that the digital
receiver is not overloaded as this will cause distortion in the
input stages of the tuner unit resulting in signal failure.

The overload problem is especially
serious due to the presence of adjacent analogue signals on the
tuner input at higher levels than the digital channel. This is
frequently made worse by the use of pre-amplifiers,

which raise both the signal and
the noise.

The most obvious way to measure
the quality of a digital transmission is to measure its bit error
rate, however in order to do this the signal must be de-modulated.
Which at present requires expensive power hungry chip sets, which
are not suited to low cost battery operated meters.

In practice, the main cause of
degradation of bit error rate (BER) in the transmission medium
is thermal noise. A large number of tests by a number of research
groups have related noise to BER. The graph below shows the relationship
between thermal noise and BER for a COFDM signal of 64 QAM.

The graph shows that the difference
in signal to noise ratio between the maximum error rate of 1E-4
and 1E-9 is only a signal to noise ratio

difference of 4.2 dB, so even
a good BER of 1E-9 only shows that we have a noise margin of
greater than 4 dB. If signal to noise is measured, a much greater
range can be obtained showing the total noise margin before failure
will occur.

A great deal of research has
shown that by far the primary cause of degradation of BER in
transmission is due to noise and that the relationship between
signal to noise ratio and BER is reliable.

Measuring the signal to noise
ratio is the quickest and most meaningful way of knowing the
margins available from the transmission.

The above considerations resulted
in the choice of signal to noise ratio as the method used to
define the quality of digital terrestrial signals in the IMdigital.
In order to make this measurement easy the process is fully automated.
The instrument selects a suitable noise floor by a scanning process.
The signal to noise is presented but in addition, the digital
readout gives the status as PASS, MARGINAL or FAIL.

Field tests carried out to date
have shown the signal to noise measurement correctly predicts
the point of failure when using digital signals.